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Variable Density Tunnel
| designated_other1_number = 114-0143 | designated_other1_num_position = bottom | image =Variable_Density_Tunnel_-_GPN-2000-001311.jpg | caption = The tank of the Variable Density Tunnel arriving in 1922 | lat_degrees = 37 | lat_minutes = 4 | lat_seconds = 43 | lat_direction = N | long_degrees = 76 | long_minutes = 20 | long_seconds = 39 | long_direction = W | location = Hampton, Virginia | nearest_city = | area = | built = 1921-1923 | architect = Max Munk | architecture = | designated_nrhp_type = October 3, 1985 | added = October 3, 1985 | established = | visitation_num = | visitation_year = | refnum = 85002795 | mpsub = | governing_body = NASA }} The Variable Density Tunnel was a wind tunnel at NASA's Langley Research Center. It is a National Historic Landmark. It was the world's first variable density wind tunnel that allowed accurate testing with small-scale models. It was actively used as a wind tunnel from the early 1920s until the 1940s. History Accurate wind tunnel testing requires matching the Reynolds number of the model with that of the actual aircraft. The Reynolds number is the ratio of inertia forces to the viscous forces in the flow. It is computed as the product of the air density, ρ'', the speed of the air relative to the aircraft, ''V, and the characteristic length scale, L'', divided by the viscosity of air, ''μ. For an airfoil, the characteristic length is usually the chord length. If a full-scale airfoil is to be simulated in a wind tunnel by a small-scale model, the Reynolds number can only be matched by increasing the velocity or its density or by decreasing its viscosity. If air is used in the wind tunnel with a small-scale model, either the speed or the density of the air must be increased. When National Advisory Committee for Aeronautics started to build a modern wind tunnel, they got Max Munk, who had studied at University of Göttingen, to design a wind tunnel using air under pressure. A large, welded-steel tank was designed with a working pressure up to 21 atmospheres. The tank was constructed by Newport News Shipbuilding and Dry Dock Company in Newport News, Virginia. The tank was long and in diameter. The tank's wall was thick. The tank required 85 tons (77.3 tonnes) of steel. The test section was in diameter to match an existing NACA Wind Tunnel No. 1, which was an open-circuit tunnel operating at atmospheric pressure. The variable-density wind tunnel had a closed-circuit design with an annular return flow to minimize the volume of the tank. A fan powered by a 250-hp motor could produce an air speed up to . is at far left.]] The tank was partially destroyed by a fire in 1927. It was rebuilt and operational in 1930, and was used up to the 1940s. Then the tank was used as a pressure tank to support other wind-tunnel activities at Langley. In 1978, the tank was taken out of service; it survives on display.NASA Langley's National Historic Landmarks The tunnel was used for over 20 years. It produced the data for 78 classical airfoil shapes that were published in 1933 in "The Characteristics of 78 Related Airfoil Sections from Tests in the Variable-Density Wind Tunnel," NACA Technical Report 460.http://www.centennialofflight.gov/essay/Evolution_of_Technology/NACA/Tech1.htm Information on NACA Technical Report 460 These data formed the basis for many World War II airfoil designs. References External links *[http://www.nps.gov/history/nr/travel/aviation/ Aviation: From Sand Dunes to Sonic Booms, a National Park Service Discover Our Shared Heritage Travel Itinerary] *NASA Langley Cultural Resources: Variable Density Tunnel Category:Wind tunnels Category:Langley Research Center Category:National Advisory Committee for Aeronautics Category:Buildings and structures in Hampton, Virginia Category:Buildings and structures on the National Register of Historic Places in Virginia Category:National Historic Landmarks in Virginia Category:National Register of Historic Places in Hampton, Virginia Category:1921 establishments in Virginia Category:Aviation-related listings on the National Register of Historic Places